Self-adhesive prepreg face sheets for sandwich panels

ABSTRACT

Resin compositions are provided which are used in combination with fibers to form self-adhesive prepreg sheets that are applied to core materials to form sandwich panels. The prepreg resin includes a thermoset resin, a curing agent and a viscosity control agent. The prepreg resin further includes certain thermoplastic particles which are used to control the flow characteristics of the prepreg resin and the formation of fillets during bonding of the prepreg to the core material.

[0001] This is a continuation-in-part application of copendingapplication Ser. No. 09/795,177, filed Feb. 27, 2001, which is acontinuation-in-part of copending application Ser. No. 09/573,760 filedMay 18, 2000.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to sandwich panels andother related structural composite materials. Sandwich panels aretypically made up of face sheets which are adhesively bonded to oppositesides of a core material to form the sandwich panel. In particular, thepresent invention relates to sandwich panels in which the face sheetsare self-adhesive so that a separate adhesive film is not required forbonding to the core.

[0004] 2. Description of Related Art

[0005] Sandwich panels are used in a wide variety of applications wherehigh strength and light weight are required. The cores which are used inmost sandwich panels are either lightweight honeycomb, rigid foam, paperor wood. Honeycomb is a popular core material because of its highstrength to weight ratio and resistance to fatigue failures. Honeycombcores are made from a wide variety of products including metals andcomposite materials.

[0006] The face sheets which are bonded to each side of the sandwichcore have also been made from a wide variety of materials which alsoinclude metals and composites. An important consideration in theformation of any sandwich panel is the adhesive which is used to bondthe face sheets to the core. The adhesive must rigidly attach thefacings to the core in order for loads to be transmitted from one facingto the other and to permit the structure to fulfill all the assumptionsimplied in the acceptance of the commonly used stress calculationmethods. If the adhesive fails, the strength of the panel is severelycompromised. The adhesive layer is especially critical in sandwichpanels which use honeycomb cores because of the relatively small surfacearea over which the honeycomb edges contact the face sheets.

[0007] Honeycomb sandwich panels are used in many applications wherestiffness and structural strength of the panel are primaryconsiderations. Additionally, honeycomb sandwich panels are also widelyused in the aerospace industry where the weight of the panel is also ofprimary importance. As a result, there has been and continues to be aconcerted effort to reduce the weight of the honeycomb sandwich panelswithout sacrificing structural strength. One area which has beeninvestigated to reduce weight is the elimination of separate adhesivelayers. This has been accomplished by making the face sheets fromcomposite materials which are self-adhesive. Exemplary self-adhesiveface sheets are described in published European Patent Application Nos.EP0927737 A1 and EP0819723 A1.

[0008] One procedure for applying face sheets to honeycomb involvesforming a prepreg sheet which includes at least one fabric or fiberlayer and an uncured prepreg resin. Prepreg is a term of art used in thecomposite materials industry to identify mat, fabric, nonwoven materialor roving which has been preimpregnated with resin and which is readyfor final curing. An adhesive is typically applied to the prepreg and itis then bonded to the honeycomb by curing of both the prepreg resin andadhesive resin at elevated temperature.

[0009] In these instances where the prepreg is bonded without using aseparate adhesive (i.e., the prepreg is self-adhesive), the prepregresin must meet the dual requirements of providing suitable structuralstrength while still providing adequate adhesion to the honeycomb. Thereis a present and continuing need to identify and develop prepreg resinswhich are suitable for use in self-adhesive prepregs to providelightweight yet structurally strong sandwich panels.

SUMMARY OF THE INVENTION

[0010] In accordance with the present invention, prepreg face sheetshave been developed that are useful as self-adhesive face sheets whichare bonded to honeycomb to form sandwich panels that are lightweight,structurally strong and exhibit many other desirable properties.Sandwich panels incorporating the self-adhesive prepregs of the presentinvention exhibit high peel strength, good hot/wet properties, lowsolvent absorption, high resistance to core crushing and otherproperties that are desirable for aerospace applications.

[0011] Self-adhesive prepregs in accordance with the present inventioninclude at least one fiber layer that is impregnated with a prepregresin to form a prepreg which has a prepreg resin layer having a bondingsurface which is bonded directly to the honeycomb during sandwich panelformation. The prepreg resin includes a thermoset resin, a curing agentand a sufficient amount of a viscosity control agent to provide aprepreg resin having a viscosity which is sufficient to allow theprepreg resin to be combined with the fiber layer to form the prepregresin layer. As a feature of the present invention, it was discoveredthat certain types and sizes of thermoplastic particles may be used asfillet forming particles that can be incorporated into the prepreg resinin amounts sufficient to make the prepreg self-adhesive while notadversely affecting the viscosity or other properties of the resin whichare required for its use as a prepreg resin.

[0012] As a further feature of the present invention, it was found thatthermoplastic fillet forming particles, such as densifiedpolyethersulfone, are not dissolved to any substantial degree when theyare loaded into the prepreg resin. As a result, the prepreg resin can beloaded with enough fillet forming particles to substantially increasethe bonding strength of the resin while at the same time not increasingor decreasing the viscosity of the prepreg resin to unacceptable levels.It was discovered that the fillet forming particles dissolve during thecuring process to provide the resin with adhesive characteristics thatenhance fillet formation between the prepreg and honeycomb. Fillet sizeand shape are known to be an important consideration in the bonding offace sheets to honeycomb. In addition, the dissolved fillet formingparticles provide toughening of the resin which improves overall bondstrength.

[0013] The present invention involves not only the uncured self-adhesiveprepregs, but also includes the prepregs after they have been attachedto honeycomb and cured to form finished sandwich panels. The inventionalso covers methods for bonding the self-adhesive prepregs to honeycombin order to form sandwich panels. The methods involve forming aself-adhesive prepreg by providing at least one fiber layer and aprepreg resin wherein the prepreg resin is combined with the fiber layerto form a prepreg having a bonding surface which is adapted to be bondeddirectly to one or both faces of the honeycomb. The prepreg resinincludes a thermosetting resin, or a combination of thermosetting resinssuch as epoxy, cyanate ester, bismaleimide, and the like, curing agentsand a sufficient amount of viscosity control agent so that the prepregresin has a viscosity which is sufficiently low to allow the prepregresin to be combined with the fiber layer to form the prepreg and yetsufficiently high to be largely retained in the fiber layer duringcuring.

[0014] In accordance with the present invention, the step of forming aself-adhesive prepreg further includes the step of incorporatingthermoplastic fillet forming particles into the prepreg resin in anamount sufficient to form a bonding surface which is self-adhesive andwherein the fillet forming particles are not dissolved to anysubstantial degree in the prepreg resin. As a final step in the method,the self-adhesive prepreg is bonded to said honeycomb wherein thebonding involves curing the self-adhesive prepreg for a sufficient timeand at a sufficient temperature to substantially dissolve the filletforming particles.

[0015] The prepregs and finished sandwich panels made in accordance withthe present invention may be used in a wide variety of situations wherea light weight and structurally strong material is needed. However, theinvention is especially well-suited for use in aerospace applicationswhere a multitude of strict mechanical and chemical requirements must bemet while at the same time not exceeding weight limitations. Theabove-described and many other features and attendant advantages of thepresent invention will become better understood by reference to thefollowing detailed description when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a perspective view of an exemplary honeycomb core andtwo self-adhesive face sheets prior to bonding together to form thesandwich panel.

[0017]FIG. 2 is a perspective view of an exemplary sandwich panel whichhas been formed by bonding together the honeycomb core and face sheetsshown in FIG. 1

[0018]FIG. 3 is a side view of a portion of the sandwich panel shown inFIG. 2.

[0019]FIG. 4 is a side schematic view showing fillet formation andparticle dissolution in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The prepregs of the present invention are preferably used to formself-adhesive faces sheets which are attached to honeycomb cores to formlight weight structural panels for use in aerospace applications.Referring to FIGS. 2 and 3, a honeycomb sandwich panel is showngenerally at 10. The sandwich panel 10 includes a honeycomb core 12which has first and second faces 14 and 16, respectively. Uncuredself-adhesive prepregs 17 and 19 (see FIG. 1) are applied to the core 12and then cured to form face sheets 18 and 20 of the finished panel 10(see FIG. 2). As shown in FIG. 3, the face sheets 18 and 20 are applieddirectly to the core 12 without a separate adhesive layer. Each of theface sheets 18 and 20 are made up of fibers 22 which are embedded in aprepreg resin matrix 24. In FIG. 1, the face sheets are shown asprepregs 17 and 19 prior to application to the honeycomb core 12.

[0021] The honeycomb core 12 can be made from any of the materials whichare used to form honeycomb cores. Exemplary honeycomb materials includealuminum, aramid, carbon or glass fiber composite materials, resinimpregnated papers and the like. A preferred honeycomb material is anaramid-based substrate, for example, NOMEX® which is available from E.I.DuPont de Nemours & Company (Wilmington, Del.). The dimensions of thehoneycomb can be varied widely. Typically, the honeycomb cores will have⅛ to ½ inch (3.2-12.7 mm) cells with the cores being ¼ inch (6.4 mm) to2 inches (50.8 mm) thick. In so far as the present invention provides a“self adhesive” prepreg, “self-adhesive” is self-defining in that theprepreg will form a suitable panel without the use of an adhesive layer.As will be discussed hereinafter, a peel test is a primary way ofverifying that the resultant panel is suitable. Preferably,“self-adhesive” prepregs yield a peel strength (under ASTMD 1781) of 20in-lb/3 in width or higher (8 pound core), and more preferably yield apeel strength of 28 in-lb/3 in width or higher (8 pound core). However,peel strength specifics are dependent on the type and size of honeycombused.

[0022] The fibers 22 which are used in the face sheets 18 and 20 can beany of the fiber materials which are used to form composite laminates.Exemplary fiber materials include glass, aramid, carbon, ceramic andhybrids thereof. The fibers may be woven, unidirectional or in the formof random fiber mat. Preferred fiber materials include 193 gsm plainweave carbon fabric with 3K fibers which are commercially available.

[0023] The resins which are combined with the fiber layer to formprepregs in accordance with the present invention include epoxy and/orcyanate ester resins, curing agents, viscosity control agents andthermoplastic fillet forming particles. Epoxy or cyanate ester resinsare first mixed with viscosity control agents to form a resin mixture.If necessary, the mixture is heated to ensure that viscosity controlagents are completely dissolved. Curing agents and fillet formingparticles are then added to the resin mixture. The final resin mixtureis kept below the temperature at which the fillet forming particlesdissolve in the resin. As a result, the fillet forming particles whichat this stage are uniformly mixed throughout the resin are not dissolvedto a substantial degree and therefore do not increase the resinviscosity to an unacceptable level. The viscosity of the resin mixtureis important because it must be such that the resin can be impregnatedinto the fiber to form the prepreg. For the purposes of thisspecification, particles which retain at least 90 weight percent oftheir original particle weight are considered to be not dissolved to asubstantial degree. Particles are considered to be substantiallydissolved when less than 10 percent by weight of the original particleremains intact within the resin.

[0024] The viscosity of the final resin mixture, including filletforming particles, should be between 150 and 1500 poise. The preferredviscosity is between 300 to 1200 poise. The preceding viscosity rangesrepresent minimum viscosities for the final resin mixture prior tomaking prepreg when said viscosity is measured by Pheometric DynamicAnalysis (Rheometrics RDA2) at settings of 2° C./min, 10 rads/sec and0.8-1.0 mm gap. The viscosity of the resin mixture gradually increaseswhen the fillet forming particles dissolve during the curing process.

[0025] During the prepregging process the fillet forming particles tendto be concentrated toward the surface of the prepreg due to inherentfiltering of the particles by the fiber layer. Alternatively, the filletforming particles may be applied (e.g., by powder deposition) to thesurface of the resin after it has been formed into a prepreg film orafter the resin has been impregnated into the fiber layer. In this way,the fillet forming particles are distributed substantially on thesurface of the prepreg. In either case, the resin temperature ismaintained at a sufficiently low level to prevent fillet formingparticles from dissolving until the prepreg is applied to the corematerial and cured.

[0026] During the curing process, the prepreg is heated to a sufficientlevel to substantially dissolve the fillet forming particles. Thedissolving of the particles during the curing process was found tocontrol the flow of resin at the prepreg-core interface to promotefillet formation. In addition, the dissolved thermoplastic particlesenhance the toughness of the bond. Fillet size and shape have long beenknown to be an important aspect of bond formation between honeycomb coreand adhesively bonded face sheets.

[0027] Referring to FIG. 4, the uncured face sheet 17 is shown inposition against the face 14 of one wall of the honeycomb 12. Theprepreg 17 includes a fiber layer 22 which has been combined with theprepreg resin to form a prepreg resin layer 30 which includes a bondingsurface 32 which is adapted to be bonded directly to the honeycomb 12 atface 14. The thermoplastic fillet forming particles 34 are shown beingdistributed preferentially on the outer faces of the face sheet 17. Itwill be noted by those skilled in the art that the fibers and particlesare not shown to scale in FIG. 4. Typically, the diameter of the fibers22 will be much smaller than the thermoplastic particles 34.

[0028] As represented by arrow 36, the prepreg 30 is cured at elevatedtemperature to form the single ply face sheet 18. As shown in FIG. 4,the cured face sheet 18 includes a cured prepreg resin matrix 38 inwhich the thermoplastic fillet forming particles are substantiallydissolved. As the fillet forming particles 34 dissolve during the curingprocess, the viscosity of the prepreg resin increases so as to formfillets 40 and 42. The fillets 40 and 42 are preferably sized so thatthe “A” dimension (parallel to the prepreg face sheet) is approximatelyequal to the “B” dimension (perpendicular to the face sheet). Further,the length of dimensions A and B are preferably maximized as much aspossible in order to achieve optimum bond strength for a given resin. Aswill be appreciated, resins having a viscosity which is too low duringthe curing process will produce fillets wherein the B dimension isrelatively large and the A dimension is very small. Such fillet shapesdo not provide good bonding. Alternatively, if the viscosity of theprepreg during curing is too high, the B dimension of the fillet isunacceptably small so that adequate bonding is not achieved. Inaccordance with the present invention, it was discovered that includingthe thermoplastic fillet forming particles in the resin matrix providesthe appropriate resin viscosity during the curing process, such thatfillets were formed where dimensions A and B were equivalent andsufficiently large to provide good bonding. Although not wishing to bebound by any theory, it is believed that the gradual dissolving of thefillet forming particles during the curing process provides a gradualincrease in resin viscosity which enhances fillet formation. Thegradually dissolving thermoplastic particles control the flow of theresin at the honeycomb surface 14 so that fillets of the type shown inFIG. 4 at 40 and 42 are formed. In addition, the prepreg resin formingthe fillets is preferentially enriched with thermoplastic that isbelieved to also contribute to increased bond strength.

[0029] Exemplary thermosetting resins which may be used to make theprepreg resin include epoxy, cyanate ester and bismaleimide resins.Exemplary epoxy and cyanate ester resins include glycidylamine typeepoxy resins, such as triglycidyl-p-aminophenol,tetraglycidyldiaminodiphenyl-methane; glycidyl ether type epoxy resins,such as bisphenol A type epoxy resins, bisphenol F type epoxy resins,bisphenol S type epoxy resins, phenol novolak type epoxy resins, cresolnovolak type epoxy resins and resorcinol type epoxy resins; cyanateesters, such as 1,1′-bis(4-cyanatophenyl) ethane (AroCy L-10, availablefrom Vantico, Inc., Brewster, N.Y.), 1,3-Bis(4-cyanateophenyl-1-1-(1-methylethylidene) benzene (RTX366, availablefrom Vantico, Inc., Brewster, N.Y.). Epoxy resins are preferred.Especially preferred epoxy blends include a mixture of trifunctionalepoxy and a difunctional bis-F epoxy.

[0030] Curing agents and viscosity control agents are also added to thethermosetting resin to form the basic prepreg resin. The curing agent ispreferably an amine curing agent and the viscosity control agent ispreferably a thermoplastic material which dissolves in the thermosettingresin.

[0031] Although the present invention contemplates the use ofthermoplastic fillet forming particles in a wide variety of prepregresins to enhance formation of fillets between the face sheets andhoneycomb core, prepreg resins based on epoxy and cyanate esterformulations are preferred. The following examples and description willbe limited to epoxy formulations with it being understood that thebonding between other prepreg face sheets and honeycomb core may beimproved by incorporating the thermoplastic fillet forming particles ofthe present invention into the prepreg face sheet. In addition, thisdescription will be limited to a discussion of sandwich panels whichhave two prepreg plies on each face of the honeycomb. The two layers arepreferably either two (0/90) plies or two (±45, 0/90) plies with warpdirection aligned with the lengthwise direction of the honeycomb. Thoseof ordinary skill in the art will recognize that the present inventionis also applicable to multiple ply face sheets which include two or morefiber layers.

[0032] Exemplary preferred prepreg resin formulations are as follows:

[0033] 1 to 70 parts by weight of an epoxy;

[0034] 5 to 40 parts by weight of an amine curing agent;

[0035] 1 to 30 parts by weight of a viscosity control agent; and

[0036] 5 to 50 parts by weight of thermoplastic fillet formingparticles.

[0037] 10 to 40 parts by weight of a trifunctional epoxy resin;

[0038] 10 to 40 parts by weight of a difunctional epoxy resin;

[0039] 11 to 25 parts by weight of an aromatic curing agent;

[0040] 0 to 3 parts by weight of a non-aromatic curing agent; and

[0041] 5 to 15 parts by weight of a viscosity control agent

[0042] 8 to 30 parts by weight of thermoplastic fillet formingparticles.

[0043] The epoxy may be composed of trifunctional epoxy, difunctionalepoxy and a wide variety of combinations of trifunctional anddifunctional epoxies. Tetrafunctional epoxies may also be used.Exemplary trifunctional epoxy include triglycidyl p-aminophenol andN,N-Diglycidyl-4-glycidyloxyaniline (MY-0510 or MY-0500 available fromVantico, Inc., Brewster, N.Y.). Exemplary difunctional epoxies which maybe used in the resin include Bis-F epoxies, such as GY-281, LY-9703 andGY-285 which are available from Vantico, Inc., Brewster, N.Y.). Bis-Aepoxies, such as GY-6010 (Vantico, Inc., Brewster, N.Y.) and DER 331(Dow Chemical, Midland, M.I.) are suitable Bisphenol-A type epoxies andmay also be used. An exemplary tetrafunctional epoxy is tetraglycidyldiaminodiphenyl methane (MY-721, MY-720 and MY-9512 available fromVantico, Inc., Brewster, N.Y.). Other suitable epoxies include phenolnovolak type epoxy, cresol novolak epoxy and resorcinol type epoxy.Preferred bis-F epoxies include GY281 and GY285 which are available fromVantico, Inc., Brewster, N.Y.

[0044] Exemplary curative agents include dicyandiamide,3,3-diaminodiphenylsulfone (3,3-DDS), amino or glycidyl-silanes such as3-amino propyltriethoxysilane, CuAcAc/Nonylphenol (1/0.1),4,4′-diaminodiphenylsulfone (4,4′-DDS),4,4′-methylenebis(2-isopropyl-6-methylaniline), e.g., Lonzacure M-MIPA(Lonza Corporation, Fair Lawn, N.J.),4,4′-methylenebis(2,6-diisopropylaniline), e.g., Lonzacure M-DIPA (LonzaCorp., Fair Lawn, N.J.). Dicyandiamide and 3,3-DDS are preferredcurative agents. Especially preferred are combinations of 3,3-DDS anddicyandiamide.

[0045] Exemplary viscosity control agents include thermoplasticpolyetherimides such as ULTEM® 1000P which is available from GeneralElectric (Pittsfield, Mass.); micronized polyethersulfone such as 5003P,which is available from Sumitomo Chemical Co., Ltd. (Osaka, Japan);HRI-1, which is available from Hexcel Corp. (Dublin, Calif.); andpolyimide MATRIMID® 9725, which is available from Vantico, Inc.(Brewster, N.Y.). ULTEM® 1000P and micronized PES are preferred.Micronized PES is especially preferred. The amount and type of viscositycontrol agent which is added to the epoxy resin mixture may be variedprovided that the minimum viscosity of the final resin mixture ismaintained between 150 and 1500 poise when said viscosity is measured byRheometric Dynamic Analysis (Rheometrics RDA2) at settings of 2° C./mni,10 rads/sec and 0.8-1.0 mm gap. As previously mentioned, mixtures withminimum viscosities of between 300 to 1200 poise are preferred. Theviscosity of the prepreg resin prior to addition of the fillet formingparticles should be between about 50 poise and 2000 poise at roomtemperature. The preferred viscosity range is 100 poise to 1500 poise atroom temperature.

[0046] Densified polyethersulfone (PES) and densified polyetherimideparticles may be used as suitable fillet forming particles. DensifiedPES particles are preferred. The densified polyethersulfone (PES)particles are preferably made in accordance with the teachings of U.S.Pat. No. 4,945,154, the contents of which is hereby incorporated byreference. The average particle size of the PES particles range from 1to 150 microns. Average particle sizes of 1 to 50 microns are preferredand average particle sizes of 10 to 25 microns are particularlypreferred. The microspheres are generally spherical in shape and areclassified by passing the densified microsphere powder through a micronsieve. It is preferred that the glass transition temperature (Tg) forthe particles be above 200° C.

[0047] In an alternative embodiment, the PES is “micronized.” MicronizedPES refers to PES particles which have a rough surface configurationwhich is produced by grinding the particles or other abrasive techniquesof manufacture which are known in the art. Micronized PES particles mayalso be made by spraying and drying procedures which are also known inthe art. Micronized PES particles are preferably less than 120 micronsin size. Especially preferred are particles under 50 microns in sizewith a range of 10 to 25 microns being particularly preferred.

[0048] The prepreg resin is made by first mixing the epoxy componentstogether and then slowly adding the polyetherimide or micronized PESviscosity control agents. The resulting mixture is heated to around 130°C. and mixed for a sufficient time to dissolve the polyetherimide/PESparticles. Once the polyetherimide/PES is dissolved, the mixture iscooled to around 75° C. The aromatic amine curing agent and the filletforming densified PES particles are then added to the mixture. The resinshould be kept at temperatures below about 70° C.-75° C. while thecurative agent and densified PES particles are being mixed into theresin. The final resin has a minimum viscosity of between 150 to 1500poise when said viscosity is measured by Rheometric Dynamic Analysis(Rheometrics RDA2) at settings of 2° C./min, 10 rads/sec and 0.8-1.0 mmgap. The preferred viscosity range is 300 to 1200 poise.

[0049] The finished resin is applied to the desired fabric to form aprepreg. The resin content of the prepreg may be varied depending upon anumber of different parameters in order to achieve desired mechanicaland structural properties for the sandwich panel. It is preferred thatthe prepreg have a resin content of 35-45 weight percent.

[0050] The prepreg is bonded to the faces of the honeycomb core usingvacuum and/or pressure and heat to cure the prepreg and form face sheetswhich are securely bonded to the honeycomb. The amount of vacuum,pressure and heat required to cure and bond the prepreg to the honeycombmay be varied depending upon the particular resin formulation and theamount of resin in the prepreg. In general, sufficient pressure must beapplied to the prepreg to ensure that the resin flows into the honeycombcells a sufficient amount to provide adequate fillet formation andbonding.

[0051] The temperature and other curing conditions are selected suchthat the densified PES particles are substantially dissolved during thecuring process. It has also been found that panels made with theinventive self-adhesive provide excellent resistance to core crush. Forexample, panels of the type set forth in Examples 1 and 2 exhibitessentially 0% core crush. Other conventional aviation panels utilizing3 K carbon fabric prepregs in accordance with the present invention willalso show improved resistance to core crushing. Even panels madeutilizing 12 K carbon fabric prepreg in accordance with the presentinvention will produce panels which exhibit only 5% core crush.

[0052] Examples of practice are as follows:

EXAMPLE 1

[0053] Resin was prepared having the following formulation:

[0054] 23 weight percent MY-0510 (N,N-Diglycidyl-4-glycidyloxyaniline)

[0055] 25 weight percent GY281 (bis-F epoxy)

[0056] 19 weight percent 3,3-Diaminodiphenylsulfone (3,3-DDS)

[0057] 7 weight percent ULTEM® 1000P (polyetherimide)

[0058] 26 weight percent densified PES

[0059] The densified PES was made from PES 5003P which is available fromSumitomo Chemical Co. Ltd. (Osaka, Japan). The PES was densified inaccordance with U.S. Pat. No. 4,945,154. MY0510 and GY281 were firstmixed in a mixing vessel, heated to 70° C. for approximately 10 minutes.The ULTEM® 1000P particles were then added and the resulting mixtureheated to 130° C. with mixing for approximately 75 minutes to fullydissolve the ULTEM® 1000P particles. The mixture was then cooled to 75°C. and the 3,3-DDS was mixed in for about 15 minutes. Then, thedensified PES was slowly added and mixed in for approximately 10 minutesto provide the final resin mixture. The viscosity of the homogeneousresin was measured over the entire curing temperature range (i.e., 20°C. to 177° C.) using Rheometric Dynamic Analysis as previouslydescribed. The resin had a minimum viscosity of 900 poise.

[0060] Panels were prepared by first forming a prepreg of 193 gsm 3K PWfabric containing 138 grams of resin square meter. The prepreg wasformed as follows:

[0061] The resin was coated on release paper by reverse roller at about175° F. (79° C.) to form a film containing 69 g/m². Two resin films wereimpregnated into the carbon fiber with an areal weight of 193 g/m².

[0062] The prepreg was applied to HRH® 10 core having ⅛ inch (0.31 cm)cells and being ½ inch (1.27 cm) thick under vacuum at 22 inches (56 cm)Hg and cured for 2 hours at 177° C. with a pressure of 45 psi, ventingat 20 psi and ramp cooling at a rate of 2° C. per minute.

[0063] The resulting specimens were subjected to peel test according toASTM D 1781. The face sheets all had peel strengths above 29 in-lb/3 inwidth. The dimensions A and B for representative fillets were measuredand found to be approximately of equal length.

EXAMPLE 2

[0064] Resin was prepared in the same manner as Example 1 except thatthe ingredients used to make the resin were as follows:

[0065] 21 parts by weight MY-0510

[0066] 21 parts by weight AcroCy® L-10

[0067] 21 parts by weight GY281

[0068] 9 parts by weight ULTEM® 1000P

[0069] 1.5 parts by weight CuAcAc/Nonylphenol (1/0.1)

[0070] 26.5 parts by weight densified PES

[0071] The minimum viscosity of the homogeneous resin mixture was foundto be about 500 poise. The viscosity of the final resin mixture wasmeasured as set forth in Example 1. The final resin mixture was used toform a prepreg and applied to HRH® 10 core in the same manner asExample 1. The peel strength of the resulting face sheet was 26 in-lb/3in width.

EXAMPLE 3

[0072] Resin was prepared having the following formulation:

[0073] 27.0 weight percent MY-0510 (N,N-Diglycidyl-4-glycidyloxyaniline)

[0074] 24.9 weight percent GY285 (bis-F epoxy)

[0075] 15.8 weight percent 3,3′-Diaminodiphenylsulfone

[0076] 1.3 weight percent Dicyandiamide

[0077] 13.5 weight percent micronized Polyethersulfone (PES)

[0078] 17.5 weight percent densified Polyethersulfone (PES)

[0079] Resin formulations in accordance with this example may also bemade wherein the amounts of MY-510, GY281 and 3,3-DDS are varied by upto +15%. Also, the amounts of both types of PES may be varied by as muchas ±40%. The amount of dicyandiamide may be varied by up to ±50%.

[0080] The densified PES was the same as used in Examples 1 and 2.Average particle size was 1025 microns with no more than 13 weightpercent smaller than 5 microns and no more than 4 weight percent greaterthan 40 microns. 24.9 parts by weight of GY285 and 6.0 parts by weightof MY0510 were mixed in a resin kettle and heated, with stirring, to 65°C. Once this temperature is attained, 13.5 parts by weight micronizedPES 5003P is added to the resin kettle. The mixture is then heated to128±2° C. and held at this temperature for 75 minutes. At the end of 75minutes, heating is removed and 21 parts by weight of MY0510 are addedto the kettle. Stirring is continued as the mixture cools to 65° C. 15.8parts of 3,3-DDS is added and mixed for 15 minutes. 1.3 parts ofdicyandiamide is then added and the mixture stirred for 5 minutes at 65°C. Finally, 17.5 parts of densified PES is added and stirred in for 10minutes. The minimum viscosity of the resin was measuared as set forthin Example 1 and found to be about 370 poise. Panels were prepared byfirst forming a prepreg of 193 gsm 3K PW carbon fabric containing 70grams of resin per square meter. The prepreg was formed as follows:

[0081] The resin was coated on release paper by reverse-roll coater atabout 165° F. (74° C.) to form a film containing 70 g/m². The resin filmwas impregnated into a carbon fiber fabric having an areal weight of 193g/m². The prepreg was then applied to HRH® 10 core and cured in the samemanner as Example 1. The peel strength was around 32 in-lb/3 in width on3 pound core and around 31 in-lb/3 in width on 8 pound core.

Comparative Example 1

[0082] Resin was prepared as follows:

[0083] Add 12.5 parts MY-0510 and 37.5 parts GY281 to a mixing vesseland heat to 70° C. for about 10 minutes. Then add 7 parts ULTEM 1000Pand heat the mixture to 130° C. Mix for about 75 minutes to fullydissolve the ULTEM® 1000P. Cool the mixture to a temperature of 75° C.and slowly add 19 parts 3,3′-DDS. Mix 15 minutes at 75° C. Finally,slowly add 26 parts densified PES and mix the resulting final mixturefor approximately 10 minutes at 75° C. The minimum viscosity of theresin was measured as set forth in Example 1 and found to be 118 poise.

[0084] Prepregs and sandwich panels were prepared in the same manner asin the preceding example. The peel strengths for the resulting facesheets were 22 in-lb/3 in width. The viscosity of the resin is believedto be responsible for the relatively low peel strength (i.e., below 25in-lb/3 in width).

Comparative Example 2

[0085] Resin was prepared following the same procedure as set forth inComparative Example 1 except that the ingredient amounts were asfollows:

[0086] 23 parts by weight MY-0510

[0087] 25 parts by weight GY281

[0088] 19 parts by weight 3,3-DDS

[0089] 4.5 parts by weight ULTEM® 1000p

[0090] 26 parts by weight densified PES

[0091] The minimum viscosity of the resin was measured as set forth inExample 1 and found to be 123 poise.

[0092] Prepregs and sandwich panels were prepared in the same manner asthe preceding examples. The peel strength for the resulting face sheetswas 20 in-lb/3 in width.

Comparative Example 3

[0093] Resin was prepared following the same procedure as set forth inthe preceding Comparative Examples except that the ingredient amountswere as follows:

[0094] 50 parts by weight MY-0510

[0095] 50 parts by weight GY281

[0096] 47.6 parts by weight 3,3-DDS

[0097] 0.0 parts by weight ULTEM® 1000p

[0098] 30 parts by weight non-densified PES

[0099] The minimum viscosity of the resin was measured as set forth inExample 1 and found to be about 30 poise.

[0100] Prepregs and sandwich panels were prepared in accordance with thepreceding examples.

[0101] The peel strength was 13 in-lb/3 in width.

Comparative Example 4

[0102] Resin was prepared following the same procedure as the previouslydescribed Comparative Examples except that the ingredients were asfollows:

[0103] 13.6 parts by weight MY721

[0104] 11.8 parts by weight MY-0510

[0105] 25 parts by weight GY281

[0106] 5 parts by weight Matrimide 9725

[0107] 20 parts by weight 3,3-DDS

[0108] 25 parts by weight densified PES

[0109] The minimum viscosity of this resin was measured as set forth inExample 1 and found to be 3187 poise. The resulting prepreg had low tackand poor draping properties because the viscosity was too high.

[0110] Having thus described exemplary embodiments of the presentinvention, it should be noted by those skilled in the art that thewithin disclosures are exemplary only and that various otheralternatives, adaptations and modifications may be made within the scopeof the present invention. Accordingly, the present invention is notlimited by the above preferred embodiments, but is only limited by thefollowing claims.

What is claimed is:
 1. A self-adhesive prepreg for bonding to ahoneycomb, said self-adhesive prepreg comprising: at least one fiberlayer; a resin which has been combined with said fiber layer to form aprepreg which includes a bonding surface that is adapted to be bondeddirectly to said honeycomb, said resin comprising a thermoset resin, acuring agent and a sufficient amount of a viscosity control agent toprovide a resin having a viscosity which is sufficient to allow saidresin to be combined with said fiber layer to form said prepreg; andthermoplastic fillet forming particles which are incorporated into saidresin in an amount sufficient to form a prepreg resin which isself-adhesive and wherein said fillet forming particles are notdissolved to a substantial degree in said prepreg resin.
 2. Aself-adhesive prepreg according to claim 1 wherein said thermoset resinis selected from the group consisting of epoxy and cyanate ester resins.3. A self-adhesive prepreg according to claim 1 wherein saidthermoplastic fillet forming particles are selected from the groupconsisting of densified and micronized thermoplastic particles whichhave a glass transition temperature that is above 200° C.
 4. Aself-adhesive prepreg according to claim 1 wherein said thermoplasticfillet forming particles are selected from the group consisting ofdensified polyether sulfone, micronized polyether sulfone and densifiedpolyetherimide.
 5. A self-adhesive prepreg according to claim 3 whereinsaid thermoplastic fillet forming particles have particle sizes rangingfrom 1 to 100 microns.
 6. A self-adhesive prepreg according to claim 1wherein said prepreg resin comprises an epoxy thermoset resin, apolyetherimide or polyethersulfone viscosity control agent and densifiedpolyether sulfone fillet forming particles.
 7. A self-adhesive prepregaccording to claim 1 wherein the minimum viscosity of said prepreg resinover the curing temperature range of said prepreg resin is between 150to 1500 poise.
 8. A self-adhesive prepreg according to claim 1 whereinthe minimum viscosity of said prepreg resin over the curing temperaturerange of said prepreg resin is between 300 to 1200 poise.
 9. Aself-adhesive prepreg according to claim 1 wherein said thermoplasticfillet forming particles are located substantially at said bondingsurface of said prepreg.
 10. A cured honeycomb panel comprising a corehaving at least one face to which a self-adhesive prepreg according toclaim 1 is bonded and wherein said thermoplastic fillet formingparticles are substantially dissolved in said prepreg resin.
 11. A curedhoneycomb panel comprising a core having at least one face to which aself-adhesive prepreg according to claim 3 is bonded and wherein saidthermoplastic fillet forming particles are substantially dissolved insaid prepreg resin.
 12. A cured honeycomb panel comprising a core havingat least one face to which a self-adhesive prepreg according to claim 5is bonded and wherein said thermoplastic fillet forming particles aresubstantially dissolved in said prepreg resin.
 13. A cured honeycombpanel comprising a core having at least one face to which aself-adhesive prepreg according to claim 6 is bonded and wherein saidthermoplastic fillet forming particles are substantially dissolved insaid prepreg resin.
 14. A cured honeycomb panel comprising a core havingat least one face to which a self-adhesive prepreg according to claim 7is bonded and wherein said thermoplastic fillet forming particles aresubstantially dissolved in said prepreg resin.
 15. A cured honeycombpanel comprising a core having at least one face to which aself-adhesive prepreg according to claim 8 is bonded and wherein saidthermoplastic fillet forming particles are substantially dissolved insaid prepreg resin.
 16. A cured honeycomb panel comprising a core havingat least one face to which a self-adhesive prepreg according to claim 9is bonded and wherein said thermoplastic fillet forming particles aresubstantially dissolved in said prepreg resin.
 17. A method foradhesively bonding a prepreg face sheet to a honeycomb comprising thesteps of: forming a self-adhesive prepreg comprising providing at leastone fiber layer and a prepreg resin wherein said prepreg resin iscombined with said fiber layer to form a prepreg resin layer comprisinga bonding surface which is adapted to be bonded directly to saidhoneycomb, said prepreg resin comprising a thermoset resin, a curingagent and a sufficient amount of a viscosity control agent so that saidprepreg resin has a viscosity which is sufficient to allow said prepregresin to be combined with said fiber layer to form said prepreg resinlayer, said step of forming a self-adhesive prepreg further includingthe step of incorporating thermoplastic fillet forming particles intosaid prepreg resin in an amount sufficient to form a prepreg layer whichis self-adhesive and wherein said fillet forming particles are notdissolved to a substantial degree in said prepreg resin; bonding saidself-adhesive prepreg to said honeycomb wherein said bonding comprisescuring said self-adhesive prepreg for a sufficient time and at asufficient temperature to substantially dissolve said fillet formingparticles.
 18. A method according to claim 17 wherein said thermoset isselected from the group consisting of epoxy and cyanate ester resins.19. A method according to claim 17 wherein said thermoplastic filletforming particles are selected from the group consisting of densifiedand micronized thermoplastic particles which have a glass transitiontemperature that is above 200° C.
 20. A method according to claim 17wherein said thermoplastic fillet forming particles are selected fromthe group consisting of densified polyether sulfone, micronizedpolyether sulfone and densified polyetherimide.
 21. A method accordingto claim 18 wherein said thermoplastic fillet forming particles haveparticle sizes ranging from 1 to 100 microns.
 22. A method according toclaim 17 wherein said prepreg resin comprises an epoxy thermoset resin,a polyetherimide or polyethersulfone viscosity control agent anddensified polyether sulfone fillet forming particles.
 23. A curedhoneycomb sandwich panel comprising a core having at least one face towhich a self-adhesive prepreg according to claim 1 is bonded and whereinsaid thermoplastic fillet forming particles are substantially dissolvedin said prepreg resin and wherein said honeycomb exhibits a core crushof less than 5%.
 24. A cured honeycomb sandwich panel according to claim23 wherein said fabric layer comprises 6K or 12K carbon fabric.
 25. Acured honeycomb sandwich panel according to claim 24 wherein said fabriclayer comprises 6K or 12 K carbon fabric and said honeycomb exhibits acore crush which is essentially 0%.